Vacuum tube operated on alternating current



Oct. 24, 1933. PHELPS VACUUM TUBE OPERATED ON ALTERNATING CURRENT Filed Feb. i927,

Patented Oct. 24, 1933 UNITED STATES VACUUM TUBE OPERATED N ALTERNATING CURRENT Boyd Phelps, Jamaica, N. Y.

Application February 16, 1927. Serial No. 168,614

Claims.

This invention relates to vacuum tube circuits and, more especially to the subject of heating the cathodes of vacuum tube detectors and amplifiers with alternating current and eliminating the 5 audible frequency output voltage fluctuations which ordinarily result from the use of alternating current for filament heating.

' Prior to the present invention it was entirely practical to use alternating current of low fre- 1o quency, such as 60 cycles, for heating the filaments of radio frequency amplifier tubes (when conditions are favorable) and for last-stage audio amplifiers. This can be done by the simple expedient of utilizing what is known as mid-point :onnections-the filaments being connected across the secondary winding of a transformer and the grid and plate return connections being made to the mid-point of the secondary winding. But for vacuum tube detectors and audioamplifiers, other than last-stage audio, the connection of grid and plate returns to the mid-points of filament supply transformer secondary windings, or equivalent arrangements, is not suificient. The reason for this is that, although the grid and plate connections may be made at the exact midpoints of the transformers, there are still low frequency fluctuations in the output circuits of the vacuum tubes due to filament temperature fluotuations resulting from the use of alternating ourrent for heating the filaments. This temperature fluctuation causes a corresponding fluctuation in the electron emission and, likewise, a similar fluctuation in the output current of the tube. In a last-stage audio amplifier the output current fluctuation due to this cause is very small in comparison to that resulting from the incoming signal and is scarcely noticeable when signals are being received. But the same degree of fluctuation in the output current of a detector tube or that of an audio amplifier other than the last stage is comparable in magnitude and often larger than the signal amplitudethe result being that the signal is smothered by the disturbing frequency.

Circuit arrangements have heretofore been devised a View to balancing out or neutralizing the so-called residual hum resulting from the temperature fluctuations of filaments operated on alternating current in the manner above referred to, but, so far as I have been able to determine,

the circuits which have been previously proposed with a view to accomplishing that object have been either inoperative from a practical standpoint or difficult to produce or adjust on a commercial scale.

or" all vacuum tubes in a radio receiver including detectors and first audio stages. The invention is, of course, also applicable to vacuum tube circuits other than receiving circuits.

The underlying pm'nciple of this invention is analogous to that of the Wheatstone bridge. A vacuum tube detector or amplifier in which the space current is caused to vary as the result of filament temperature fluctuations is treated as one ratio arm of a Wheatstone bridge while a sec ond tube,.the function of which is to offset the effect of those fluctuations, is treated as an opposing arm of the bridge. The filament of the second tube is heated from the same source of alter nating current as that of its companion detector or amplifier tube and, consequently, its space current path resistance rises and falls in step with that of the detector or amplifier tube. If the two tubes have similar characteristics so that the internal plate-filament resistances vary equally or in the same percentage relation it is evident that the bridge will remain balanced throughout the entire cycle of filament current, and, as a result of that constant balance, there is practically a total absence of voltage fluctuation across the output terminals of the detector or amplifier tube due to filament temperature fiuctuations.

The theory of this invention and its practical application will be discussed and described hereinafter in conjunction with the accompanying drawing in which Fig. l is a schematic diagram illustrating the relation of the invention to the Wheatstone bridge and, Fig. 2 is a diagram of a radio receiving circuit which will serve as an example of a practical application of the invention.

Referring now to the schematic diagram of Fig. 1, vacuum tubes 1 and 2 are included respectively in two ratio arms of a Wheatstone bridge circuit arrangement. The filaments 3 and 4 of the tubes 1 and 2 respectively are heated with alternating current from a common source. These filaments are shown connected across the tern1inals of transformer secondary windings 5 and 6 respectively. Assuming that the plate-filament resistances of the two vacuum tubes 1 and 2 are equal and further assuming that the resistances of the two plate-filament space paths rise and fall tion of balance exists and that there is no difference of potential between the points X and Y at any instant during the filament heating cycle. The two batteries '7 and 8 are assumed to be of equal voltage for the present purpose to illustrate the Wheatstone bridge analogy. It follows then that the current flow from battery '7 through coil 9 is exactly equal and opposite to the current flow from battery 8 through coil 9. There is therefore normally no potential difference between points X and Y. When the temperature of the two filaments 3 and 4 falls during that portion of the heating current cycle when the applied voltage is falling, the plate-filament space resistance of both tubes increases equally. Likewise when the filament temperature rises the plate-filament space resistance decreases equally. The bridge balance is accordingly unaffected by the alternating filament heating current.

Tube 1 of Fig. 1 may be utilized as a detector or amplifier tube, the signal input being impressed across the filament and grid as indicated in the diagram. It is apparent that any change of potential on the grid of tube 1 caused by an incoming signal will cause corresponding changes in the plate-filament resistance of that tube but tube 2 is not correspondingly affected. It will be seen that coil 9 is in the output circuit of tube 1 and it will also be seen that any fluctuations in the plate-filament resistance of tube 1 which is not accompanied by a corresponding fluctuation in tube 2 will result in a fluctuating current flow through coil 9. The incoming signal waves impressed across the input terminals of tube 1 are detected or amplified by that tube and are present in coil 9 which may be the primary winding of either a radio frequency or audio frequency transformer as indicated. The secondary winding of this transformer marked Output may be connected to a succeeding amplifier or detector as the case may be or a reproducing device.

A practical application of the invention illustrated in Fig. 1 is shown in the circuit diagram of Fig. 2. In Fig. 1 two batteries 7 and 8 are shown, in order to simplify the explanation and to show more clearly the analogy to a bridge circuit. In the practical application of the invention it is not necessary to use two batteries as in Fig. 1.

The circuit of Fig. 2 is a radio receiving circuit comprising a vacuum tube detector and two stages of audio frequency amplificationall the filaments being heated with alternating current from a common source. Vacuum tube 10 is a detector, 11 is the first audio amplifier and 12 is the second audio amplifier. The filaments 13, 14 and 15 are connected in parallel to the secondary winding 16 of a low frequency transformer which is connected to an alternating source of say cycles. The input circuit of detector tube 10 includes coil 17 and tuning condenser 18 while the output circuit of that tube includes primary winding 19, blocking condenser 20 and battery 21. Both the input and output circuits are connected to the mid-point 22 of the secondary winding 16 for the purpose of maintaining the grid and plate at zero potential with respect to the alternating filament heating current. Between the positive terminal of battery 21 and the plate terminal of tube 10 there is shown connected a two element vacuum tube 23 having a plate and a filament. The latter tube is analogous to vacuum tube 2 of Fig. 1. Direct current cannot flow from battery 21 through winding 19 on account of the blocking condenser 20. Therefore all the plate current which flows through detector tube 10 mustalso flow thru tube 23. Considering the space paths of tubes 10 and 23 as resistances it is evident that there is normally a steady potential difference between the output terminal of the plate of detector tube 10 and the positive terminal of battery 21. t is also evident that if the space paths of these tubes are equal and vary equally and synchronously that there is no resultant change of potential between the points referred to. The filaments of tubes 10 and 23 being heated from the same source of alternating current their temperatures rise and fall together and by analogy to the explanation given in connection with Fig. 1 there is no resultant voltage fluctuation across the terminals of winding 19.

When an incoming signal wave is impressed on the input terminals of detector tube 10 the space resistance of that tube is caused to fluctuate in the usual manner but since the space resistance of tube 23 is not likewise affected there is a voltage fluctuation between the positive terminal of battery 21 and the plate terminal of detector tube 10. correspondingly there is a current fluctuation, due to the signal, in the primary winding 19. The tube 10 will function as a detector in the usual manner. The grid or control electrode of tube 16 can be biased if it is found desirable to do so by inserting a grid biasing battery in the grid circuit or by any other known means for obtaining a suitable bias.

It is not necessary that the space resistance of tubes 10 and 23 be equal. It is only essential that the resistance variations caused by the cyclical "lament temperature variations be substantially in the same proportion to the normal space resistances of the tubes. In other words it is the percentage variation that controls. Obviously better results are apt to be obtained when the platefilament characteristics of tubes 10 and 23 are alike although this is not necessarily the case in all instances.

Since the tubes 10 and 23 are connected in series it is necessary that the voltage of battery 21 be twice that required for tube l0assuming that the space resistances are equal. For example if it requires 45 volts for the best operation of tube 10, battery 21 would ordinarily be of 90 volts. This would give 45 volts drop across tube 23 and 45 volts drop across tube 10.

In the first stage of audio amplification following the detector the same arrangement is provided for eliminating the residual Tube 11 is the first audio amplifier and tube 24 is its associated balancing tube. The function of tube 24 is exactly the same as that of tube 23. It will be seen that the circuit of the first audio stage is substantially identical with that of the detector except for the showing of a grid biasing battery 25. Plate battery 26 is in series with battery 21 and is also in series with the space paths of tubes 11 and 24. It is common practice to apply 90 Volts to the plate of an audio amplifier but with tubes 11 and 24 in series it is necessary to provide twice the plate voltage in order to get the required 90 volts on the plate of the amplifier tube. If batteries 21 and 26 are each of 90 volts the required 180 volts is supplied by the two batteries in series.

In the last stage of audio amplification the tube 12 has its filament connected across the secondary winding 16 but no provision is made in this stage for eliminating the residual hum as its magnitude is negligible in comparison with the signal strength. It is obvious that the same arrangement as before described could be applied to the last stage of audio amplification for cancelling out the residual hum.

Although in the circuit diagram of Fig. 2 batteries have been indicated as sources of plate current it is obvious that these can be replaced by any of the well known devices for converting house lighting current, either alternating or direct, into continuous direct current of the proper voltage, in which event the circuit is operable entirely from any of the usual house lighting current sources.

I have chosen to illustrate my invention in a circuit including only a detector and two stages of audio frequency amplification but it is evident that the invention is applicable to more complex circuits and that it may be applied to radio frequency amplifiers in the same manner as described for, audio amplification and detection.

It is also apparent that the principles of the invention herein described may be applied in many different embodiments. The scope of this invention is therefore not to be considered as limited in any sense to the specific disclosure herein contained.

What I claim is:

1. In a wave signaling system, a three electrode vacuum tube comprising a plate, a grid and a cathode, an input circuit connected to said grid and cathode, a source of fluctuating current for heating said cathode, a source of continuous current connected in circuit with said plate and cathode, a second vacuum tube comprising a plate and cathode, the space path of said second tube being connected in series with said source of continuous current and the space path of said first vacuum tube, the cathode of said second tube being heated by fluctuating current from said source of fluctuating current, and an output path connected in parallel with the space paths of both said vacuum tubes.

2. A vacuum tube circuit comprising a three electrode vacuum tube having a filament, a plate and a grid, a source of alternating filament supply current connected to said filament, an input circuit and an output circuit for said tube, said input circuit and output circuits being connected to said filament through a point of intermediate potential with respect to the alternating current filament supply, a source of direct current for supplying the output circuit of said tube, and a second vacuum tube having a filament and a plate, said last mentioned filament and plate being connected with the space path therebetween in series with said source of direct current and the space path of said first vacuum tube, the filament of said last mentioned vacuum tube being connected to said source of alternating filament supply current, and output terminals for said first vacuum tube, said output terminals having connected between them the space path of said second vacuum tube.

3. In combination, a three electrode vacuum tube having. a plate, a grid and a filament, a source of alternating current connected to said filament, a source of direct current for supplying plate-filament space current to said tube, an impedance in series with said source of direct current and the plate-filament space path of said tube, and means responsive to the alternating filament supply current for varying said impedance synchronously and proportionately to the variations of the space current in said tube caused by filament temperature fluctuations, and output terminals for said tube, said impedance being connected between said output terminals.

4. A circuit comprising a vacuum tube having a cathode, a plate and a control electrode, a space current supply circuit including said plate, said cathode and a space current source of substantially constant potential, a source of fluctuating current for heating said cathode, a second vacuum tube having a cathode and a plate, the plate-cathode space path of said second tube being included in series in the space current supply circuit of said first vacuum tube, the cathode of said second vacuum tube being connected to said source of fluctuating current and heated thereby.

5. A circuit comprising a plurality of vacuum tubes each having anodes and cathodes, a source of fluctuating current for heating said cathodes, a substantially constant potential as a space current source, circuit connections between an anode of one of said vacuum tubes and a cathode of another of said vacuum tubes and said space current source such that space current flowing in one of said vacuum tubes is through the space path of another of said vacuum tubes opposing effects caused by cathode temperature variations to provide a constant potential drop between the anode and cathode of one of said vacuum tubes.

BOYD PHELPS. 

